1. Field of the Invention
The present invention relates to the field of magnetic resonance imaging (MRI) equipment, and in particular to a superconducting magnet device and a magnetic resonance imaging system comprising the same.
2. Description of the Prior Art
The superconducting magnet device in a magnetic resonance system requires a refrigerant—for example, liquid helium—to cool the superconducting coils to a temperature lower than the critical point. To this end, when such a superconducting magnet device is being manufactured, the superconducting wire must be dry-wound or wet-wound on a former. In general, multiple coils must be wound to maintain a steady homogeneous field strength. The coil(s) formed by winding is/are impregnated with epoxy resin in a vacuum chamber, the coil(s) is/are then placed in a low-temperature vessel in the vacuum chamber, and refrigerant is injected into the low-temperature vessel so that the coil(s) is/are immersed in the refrigerant.
FIG. 1 is a structural diagram of an existing superconducting magnet device; this superconducting magnet device adopts a structure which is currently in widespread use, having inner-layer and outer-layer coils and formers. A plurality of inner-layer coils 4 are wound on an inner-layer former 1, a plurality of outer-layer coils 3 are wound on an outer-layer former 2; a frame 5 is located between the inner-layer former 1 and the outer-layer former 2, the purpose thereof being to assemble the two formers and support the outer-layer former 2 such that this is fixed relative to the inner-layer former 1.
When a quench occurs in a superconducting coil during production or operation of a superconducting magnet device, Joule heating arising from the current in the superconducting wire and the eddy currents induced in the former and/or low-temperature vessel will cause the refrigerant surrounding the superconducting coil to rapidly boil and vaporize, in which case the volume of the refrigerant expands sharply. Since the low-temperature vessel's ability to discharge gas in a short period of time is limited, the pressure in the low-temperature vessel (hereinafter referred to as the “quench pressure”) will at this point rise rapidly.
Obviously, an excessively high quench pressure presents a serious risk to the safety of the low-temperature vessel and even the reliability of the superconducting magnet device. The following three solutions have already been proposed in the prior art for the elimination of this risk:
1) Reinforcement of the low-temperature vessel, so that it is able to withstand a higher quench pressure. The disadvantage of this solution is that it increases costs, because more stainless steel or aluminium alloy material is generally needed to reinforce the structure of the low-temperature vessel.
2) Increasing the size of the exhaust channel and exhaust port. The disadvantage of this solution is that the dimensions of the exhaust channel and exhaust port are limited by the installation space, and blindly increasing the size of the exhaust channel and exhaust port (especially when a relatively large neck tube design is used in the superconducting magnet device) will increase the additional heat load of the low-temperature vessel.
3) Making the formers from an aluminium alloy with lower resistivity and thermal conductivity. This solution is capable of lowering the quench pressure effectively, but the cost of using such an aluminium alloy is relatively high.